1. Field of the Invention
The present invention relates to a photographic lens system suited for use with lens shutter cameras and so on, and more specifically a wide-angle photographic lens system which has an F number on the order of 2.8 or a high aperture ratio and high optical performance.
2. Description of the Prior Art
The conventional photographic lens system each of which has a single focal point and is configured for use with the lens shutter cameras, etc. generally have field angles on the order of 60.degree. to 64.degree.. Though the triplet type is known well out of the three types of the conventional photographic lens systems, the triplet type photographic lens systems have F numbers which find a limit around 3.5. For this reason, those skilled in the art often select, for photographic lens system for use with the lens shutter cameras, the telephoto type and Tessar type which can have aperture ratios on the order of F/2.8. Known as conventional examples of the telephoto type photographic lens systems are lens systems which are disclosed by Japanese Patents Kokai Publication No. Sho 56-91,206, Kokai Publication No. Sho 57-116,313 and Kokai Publication No. Sho 59-147,312. Further, there are known a large number of conventional examples of telephoto type photographic lens systems other than those disclosed by the Japanese patents mentioned above. Each of these conventional photographic lens systems consists of a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power. This photographic lens system was originally developed as a telephoto lens system, but is currently applied as a wide-angle photographic lens system since this lens system has a merit that it permits locating a principal point thereof at a position close to an object point therefor, whereby this lens system can be configured so as to have a short total length and a telephoto ratio around 1. However, the telephoto type photographic lens system produces astigmatism, curvature of field and distortion in amounts too large for correction in practice. Therefore, the conventional photographic lens systems disclosed by the Japanese patents mentioned above adopt aspherical surfaces on the lens components disposed on the image side for correcting these aberrations. Further, the other conventional photographic lens systems use aspherical surfaces in the front lens units and/or rear lens units for correcting aberrations.
Known as the conventional Tessar type photographic lens systems are lens systems which are disclosed by Japanese Patents Kokai Publication No. Sho 60-176,011 and Kokai Publication No. Hei 2-208,616. Each of these conventional Tessar type photographic lens systems selects a composition in which an aperture stop is disposed on the image side of the lens system for simplifying a mechanism to move the lens units for focusing the photographic lens system. Since the Tessar type photographic lens system has a principal point located therein, this type of photographic lens system has a total length larger than that of the telephoto type photographic lens system and is disadvantageous for configuring a compact camera which is to use this photographic lens system.
Furthermore, a lens system which was disclosed by Japanese Patent Kokai Publication No. Sho 64-90,409 is known as a lens system configured as a modified version of the triplet type photographic lens system. The photographic lens system disclosed by this Japanese patent is a lens system composed of four lens components of four lens elements which are obtained by dividing a third lens component of the triplet type photographic lens system, and has an aperture ratio of F/2.8. Stops are disposed in certain photographic lens systems which are preferred as embodiments of the lens system disclosed by the Japanese patent mentioned above.
On the other hand, known as a conventional example of photographic lens system for use with single-lens reflex cameras is a lens system which was disclosed by Japanese Patent Kokai Publication No. Sho 55-105,216. This photographic lens system consists of the following: (i) a front lens unit which is composed of, in order from the object side, a positive lens component, a positive lens component and a negative lens component; and, (ii) a rear lens unit composed of a positive lens component. That is to say, this photographic lens system consists of four lens components of four lens elements or four lens components of five lens elements, and has a focal length of 40 mm and an aperture ratio of F/3.5.
For configuring a lens shutter camera such that it is compact, it is necessary to shorten a total length of a photographic lens system (a length as measured from a first surface of the lens system to a film surface) which is to be used with the lens shutter camera. A shorter total length of the photographic lens system makes it possible to configure the camera so that it has smaller thickness as a whole and higher optical performance. For this reason, most of the conventional photographic lens systems described above are configured so as to have total lengths as short as possible.
In recent years where lens barrels can be manufactured with high precisions and so as to have sophisticated functions, but cameras are made compacter not only by shortening the lens barrels but also by configuring photographic lens systems so that they can be accommodated into camera bodies. Thickness of a camera, in a condition where it is collapsed, is determined dependently on a total length of a photographic lens system built therein and a distance (.SIGMA.d) as measured from a first surface to a final surface of the photographic lens system. For configuring a camera compacter, it is therefore necessary to shorten both the total length and .SIGMA.d of a photographic lens system which is to be used with the camera.
For the reason described above, the telephoto type photographic lens systems which have large values of .SIGMA.d are unsuited for use with the collapsible mount type cameras.
Further, the Tessar type photographic lens systems which have small values of .SIGMA.d are suited for use with the collapsible mount type cameras, but insufficient in optical performance thereof since these photographic lens systems produce astigmatism and curvature of field in amounts too large for correction even by using aspherical surfaces.
The photographic lens system which was disclosed by Japanese Patent Kokai Publication No. Sho 64-90,409 has a small value of .SIGMA.d and is advantageous, like the Tessar type photographic lens systems, for use with the collapsible mount type cameras, but produces astigmatism and curvature of field in large amounts and is insufficient in optical performance thereof.
Though the photographic lens system disclosed by Japanese Patent Kokai Publication No. Sho 55-105,216 is configured for use with the lens shutter cameras, this lens system is unsuited for use with the collapsible mount type cameras since the lens system has a large F-number and a large value of .SIGMA.d.